Refers to the morning of the current day. Can be used as a noun or adverbially without a particle.
今朝は早く起きた。
I got up early this morning.
今朝、駅で友達に会った。
I met a friend at the station this morning.
There was an earthquake early this morning.
I've had a headache since this morning.
Kana spelling is common in informal writing and for learners.